Metal nanoparticles, whose particle diameter is less than 100 nm, are very different from ordinary particles in terms of their characteristics. For instance, with gold (Au), it is known that once the particle diameter drops below 10 nm, the sintering temperature drops to under 200° C., which is far lower than the melting point.
Because their characteristics include catalytic action, these metal nanoparticles have promising applications in a variety of fields in the future. Anticipation is particularly keen regarding the use of metal nanoparticles as the main component of materials used to form electronic wiring, because of the need for higher speed and density in electronic components. In this regard there have been studies aimed at practical applications that would take advantage of the low temperature sinterability of metal nanoparticles in polyimide or ordinary organic substrates, rather than just materials such as ceramic or glass that have been used up to now.
One of the known methods for manufacturing metal nanoparticles is to obtain metal nanoparticles in the vapor phase by evaporating the raw material metal under a vacuum and in the presence of a small amount of an expensive inert gas such as helium.
According to this method, however, the amount of metal nanoparticles produced at one time is generally small. Also, because an apparatus such as an electron beam, plasma generator, laser, or inductive heater is required in order to evaporate the metal, this method is not very well suited to mass production. Furthermore, a drawback to metal nanoparticles obtained by these vapor phase methods is that when they are taken out as a solid, they tend to agglomerate.
A method for preparing metal nanoparticles from the liquid phase has also been proposed as an alternative to the above-mentioned vapor phase methods. For instance, one known method is to manufacture metal nanoparticles by reducing an ammoniacal silver nitrate complex solution in a hydrophobic reaction tank. Nevertheless, even metal nanoparticles obtained by a liquid phase method have a relatively strong tendency to agglomerate.
Also, these methods, almost without exception, require the addition of a surfactant to form a protective colloid in order for the particles to remain dispersed stably, but even so there is room for improvement in terms of dispersion stability.